The objective of this project is to increase our knowledge of the antigens that are present on the cell surface of Streptococcus mutans since the knowledge is an essential basis for immunoprophylactic studies related to dental caries. Specifically the rhamnose-glucose polymers (RGP), present in the cell walls of S. mutans serotypes a, d, and g strains, are being studied. These wall antigens have not been previously characterized. To date, RGP has been purified from cell walls of S. mutans strain B13 (d) and chemically and immunologically characterized. The purified RGP/B13 contained 90% carbohydrate, 1.4% protein and 0.16% phosphorus. Analysis by gas chromatography indicated that RGP/13 was composed of rhamnose and glucose in a 1.6:1 ratio. RGP/B13 was immunogenic in rabbits when animals were immunized with whole cells or cell walls. RGP/B13 was both chemically and immunologically distinct from the serotype d antigen. In further studies, RGP/13 was shown to be immunologically identical to RGP purified from S. mutans 6715-T2 (serotype g). Initial studies also showed the purified RGP/B13 to be cross-reactive with three antisera to S. mutans strain Ingbritt, serotype c. The wide cross-reactivity of RGP may be useful in the development of a preventative dental caries vaccine which has a broad spectrum of activity.